Signal transmission apparatus and a method of signal transmission

ABSTRACT

An apparatus and method of transmitting a plurality of polarized signals having different wavelengths through a length of signal transmission cable which changes the polarization of the signals in correspondence with their amplitudes and wavelengths. A dynamic filter capable of differential adjustment of the amplitudes of the signals and a polarizing element are provided in series with the length of signal transmission cable and the amplitudes of the different wavelength signals are so adjusted that the polarization states of the signals move towards alignment with the low-loss axis of the polarizing element. An advantage provided by the apparatus and method is that the high-amplitude signals are aligned with the low-loss axis of the polarizing element while low-amplitude noise is aligned with the high-loss axis of the polarizing element, resulting in an improvement in signal-to-noise ratio.

The invention relates to signal transmission apparatus and a method ofsignal transmission, especially optical signal transmission apparatusand a method of optical signal transmission.

Signals travelling in an optical fibre experience a rotation ofpolarisation arising from the natural birefringence of the opticalfibre. In wavelength division multiplexed (WDM) signal transmission inan optical fibre, the optical signals are rotated in polarisation atdifferent rates. An additional polarisation rotation effect, which isnon-linear and due to the Kerr effect, is also present and results ininstantaneous changes in the refractive index in one polarisation and,hence, the birefringence, of the optical fibre, depending on the powerof the optical signal.

An object of the present invention is the control of the extent ofrotation of the polarisation of the signals in a signal transmissioncable such as an optical fibre in WDM transmission, reducing transmittednoise.

The invention provides signal transmission apparatus including:

a signal source capable of generating a plurality of polarised signalshaving different wavelengths,

controllable gain-control means, including a dynamic filter capable ofdifferential adjustment of the amplitudes of the signals, having asignal-input port connected to receive signals from the signal sourceand a signal-output port for delivering an amplitude-controlled signal,the controllable gain-control means having a control-input port throughwhich the gain of the dynamic filter is adjustable for respective onesof the plurality of signals,

a length of signal transmission cable including an input port at one endand an output port at the other end, the input port of the length ofsignal transmission cable being connected to receive signals from thesignal-output port of the controllable gain-control means, the length ofsignal transmission cable, in operation, changing the polarisation ofsignals passing through it in accordance with the amplitudes andwavelengths of the signals,

a polarising element having a signal-input port connected to receivesignals from the output port of the length of transmission cable and asignal-output port for delivering, to a receiving device, signalsconveyed from the length of transmission cable and

a signal monitoring device capable of determining the respective statesof polarisation of a plurality of signals present at its signal-inputport which is connected to receive signals from the signal-output portof the polarising element, the signal monitoring device having an outputport connected to deliver, to the control-input port of the controllablegain-control means, the result of the determination which, in operation,it makes for the signals it receives

the signal monitoring device and the controllable gain-control means sooperating together as to move the polarisation states of the signals, bycontrolling the amplitudes of the signals as they pass through thecontrollable gain-control means, towards alignment with the low-lossaxis of the polarising element.

Preferably, the controllable gain-control means includes an intermediatepower control device having an input port which serves as thecontrol-input port of the of the controllable gain-control means, anoptical amplifier and a dynamic gain control element which includes thedynamic filter, the intermediate power control device being connected tocontrol the optical amplifier and the dynamic gain control element inaccordance with the signals received from the signal monitoring device.

Preferably, the polarising element is a polarising element for whichattenuation on the high-loss axis is greater, but not substantiallygreater, than attenuation on the low-loss axis, preferably, the lengthof transmission cable is a length of optical transmission cable and,preferably, the signal source is capable of generating return-to-zerosignals.

The signal transmission apparatus may include:

a further length of signal transmission cable including an input port atone end and an output port at the other end,

a further controllable gain-control means having a signal input portconnected to receive signals from the signal-output port of thepolarising element and a signal-output port for delivering furtheramplitude-controlled signals to the input port of the length of signaltransmission cable, the further controllable gain-control means having acontrol-input port,

a further polarising element connected to receive signals from theoutput port of the further length of signal transmission cable and

a further signal monitoring device capable of determining the respectivestates of polarisation of a plurality of signals present at itssignal-input port which is connected to receive signals from thesignal-output port of the further polarising element, the signalmonitoring device having an output port connected to deliver, to thecontrol-input port of the further controllable gain-control means, theresult of the determination which, in operation, it makes for thesignals it receives

The invention also provides a method of transmitting a plurality ofpolarised signals having different wavelengths through a signaltransmission cable which changes the polarisation of the signals incorrespondence with their amplitudes and wavelengths, including thesteps of providing dynamic filter capable of differential adjustment ofthe amplitudes of the signals and a polarising element in series withthe signal transmission cable, and so adjusting the amplitudes of thedifferent wavelength signals, at the dynamic filter, that thepolarisation states of the signals move towards alignment with thelow-loss axis of the polarising element.

Preferably, the method includes the step of monitoring the states ofpolarisation of the signals leaving the polarising element andcontrolling the amplitudes of the signals at the dynamic filter inaccordance with the results of the monitoring step.

Preferably, the method includes the steps of generating return-to-zerosignals (RZ) signals and transmitting the RZ signals.

A signal transmission apparatus and a method of signal transmission inaccordance with the invention will now be described, by way of exampleonly, with reference to the accompanying drawing which shows an opticalWDM transmission apparatus.

Referring to the accompanying drawing, the signal transmission apparatusincludes a transmitter 1 connected to supply optical signals to apolarisation controller 2. The polarisation controller 2 is connected tosupply signals to a signal-input port of an optical amplifier 3, thesignal-output port of which is connected to supply signals to asignal-input port of a dynamic gain control element 4. The dynamic gaincontrol element 4 has an output port which is connected to the inputport of a length of optical fibre cable 5, the output port of which isconnected to a signal-input port of a polarising element 6. Thepolarising element 6 has a signal-output port which is connected to asignal-input port of a receiver 9. The output port of the polarisingelement 6 is also connected to a signal-input port of asignal-monitoring device 7 which is capable of monitoring the state ofpolarisation of the signals which it receives from the polarisingelement 6 and the output port of the monitoring device 7 is connected toan input port of an intermediate power control device 8. Theintermediate power control device 8 has a first output port connected toa control input port of the dynamic gain element 4 and a second outputport connected to a control input port of the optical amplifier 3, tocomplete the signal transmission apparatus.

The optical amplifier 3, the dynamic gain control element 4 and theintermediate power control device 8 function as a controllablegain-control means 10 and includes a dynamic filter capable ofdifferential adjustment of the amplitudes of the signals. The controlinput port of the intermediate power control device 8 serves as thecontrol input port of the controllable gain-control means 10 and thegain of the dynamic filter is adjustable for respective ones of theplurality of signals, through the control input port. The intermediatepower control device 8 is an electrical circuit and the connections toand from it are purely electrical. The gain of the controllablegain-control means 10 is, in fact, a controllable wavelength-dependentloss.

The signal-monitoring device 7 includes a polarisation-splitting element7 a and power monitors 7 b and 7 c which detect the power in the twoplanes of polarisation into which the light is split. The two planes ofpolarisation referred to correspond to the low-loss and high-loss axes,respectively, of the polarising element 7.

In the operation of the signal transmission apparatus, the transmitter 1generates a plurality of optical signals at different wavelengths whichare subjected to controlled polarisation by the polarisation controller2 which is downstream of the transmitter 1. The signals pass from thepolarisation controller 2 to the optical amplifier 3 which amplifies thesignals. The transmitter 1 and the polarisation controller 2 serve as asource of polarised signals having different wavelengths and ofadjustable polarisation for providing signals within the dynamic rangeof the remainder of the apparatus. The signals pass from the opticalamplifier 3, which provides a controllable gain, to the dynamic gaincontrol element 4 which provides a controllable wavelength-dependentloss, the amplitudes of the signals being adjusted in the amplifier 3and the dynamic gain control element 4 in accordance with a controlsignal applied by the intermediate power control device 8 to thecontrol-input ports of the amplifier 3 and the dynamic gain controlelement 4. The control signal provided by the intermediate power controlelement 8 is developed from the signal provided by the signal monitoringdevice 7 and is such that the amplitudes of the plurality of signalsconveyed by the dynamic gain control element 4 are adjusted to cause thepolarisation states of the signals to move towards being aligned withthe low-loss axis of the polarising element 6. The amplitude-adjustedsignals leave the dynamic gain control element 4 and enter the length ofoptical transmission cable 5 in which the plane of polarisation of anoptical signal is subjected to rotation in accordance with the square ofthe amplitude of the optical signal before reaching the polarisingelement 6. The signal monitoring device 7 examines the signals at theoutput port of the polarising device 6 along the low-loss and high-lossaxes of the polarising element 6 and provides signals to theintermediate power control device 8, for effecting the amplitude controlof the signals passing through the amplifier 3 and the dynamic gaincontrol element 4. The signals leaving the polarising element 6 are alsoavailable to the receiver 9.

The dynamic gain control element 4 serves to provide a controllablewavelength-dependent loss for the signals and may be realised in theform of a dynamic gain flattening filter (DGFF) so operating as toadjust the amplitudes of the signals in accordance with the controlsignal provided by the intermediate power control device 8. A devicesuitable for inclusion in the dynamic gain control element 4 isdescribed under the heading “Liquid-Crystal Optical Harmonic Equalizers”in Paper We.P.38 of the conference ECOC-01 held in Amsterdam.

The polarising element 6 is a weak polarising element, transmittingsignals on its high-loss axis with greater, but not substantiallygreater, attenuation than signals on its low-loss axis. Signals shouldbe detectable on both the high-loss and low-loss axes of the polarisingelement 6 and that requirement, in effect, determines the extent towhich the element 6 is a polarising element.

The signal monitoring device 7 monitors the power of the signals andseparates the signal energy into two planes of polarisation, onepolarisation aligned with the low-loss axis of the polarising element 6and the other polarisation aligned with the high-loss axis of thepolarising element 6. The energy in both planes of polarisation isrecorded and the state of polarisation of the signals is determined forthe provision of the signal to the intermediate power control device 8.

The signals are, preferably, return-to-zero (RZ) signals for which thesignal transmission apparatus is especially suitable in that there isprovision of noise reduction. The apparatus permits the plane ofpolarisation of the high-amplitude signal pulses to be aligned with thelow-loss axis of the polarising element 6 whereas any accompanyingnoise, which is at a relatively low amplitude, is aligned with thehigh-loss axis of the polarising element 6 and is suppressed.

Through appropriate use of the polarisation and amplitude control,optical data-carrying return-to-zero signals may be made to propagateover vast distances without appreciable loss of data. As a result ofnon-linear polarisation rotation, whereby the polarisation of a signalrotates as it passes down the optical fibre 5 in proportion to thesquare of its amplitude (NPR), high-intensity pulses are rotated by adifferent amount from the low-intensity noise. By using the weakpolariser 6 and the polarisation controller 2, the signal is so alignedthat the signal pulses pass through the polariser 6 in a state of lowattenuation and the majority of the noise is subjected to higherattenuation. In that way, the noise is suppressed and only the signalpasses.

Not only is the optical signal to noise ratio (OSNR) improved but, bysuppressing the noise, Gordon-Haus jitter is also reduced. Additionally,as the polarisation rotation in the optical cable 5 is proportional tothe square of the amplitude, the pulse is reshaped, providing additionalrobustness to polarisation mode dispersion (PMD) and chromaticdispersion. In effect, the optical cable 5 behaves as a saturableabsorber. The amount of static gain rotation will fluctuate over timedue to mechanical and thermal effects, and so the launch power requiredto pass through the element in the correct state must be controlleddynamically from the polarising element 6 located at the receiver 9(which may be at a remote location) or by a plurality of polarisingelements positioned periodically along the transmission line. Thecontrol is effected by taking a tap from the optical signal andmonitoring the power on both arms of the polarising beam splitter 7 a,one arm aligned with the low loss axis of the polarising element. Byrecording the total power on both arms and the ratio between the powerin each arm, the state of polarisation of the signal channels isascertained and fed back to the transmitter end.

The wavelengths are rotated at different rates, in the wavelengthdivision multiplexed (WDM) system, as the signals propagate along theoptical cable 5 and is dealt with by controlling the amplitudes ofindividual channels on a channel by channel basis. This is achievedsimply and cheaply by using a dynamic gain-flattening filter (DGFF).Instead of attempting to flatten the amplitude profile of the signal,the DGFF so adjusts the amplitudes of the signals as to align thechannels to the polarisation states of the polarising element 6.

The overall length of the signal transmission line included in thesignal transmission apparatus may be provided by a plurality of lengthsof optical cable with respective controllable gain-control means,polarising elements and signal monitoring devices connected, in serieswith one another.

The invention is applicable to all signals in cases where the plane ofpolarisation of the signal is rotated by a transmission cable alongwhich the signal travels.

1. A signal transmission apparatus, comprising: a) a signal source forgenerating a plurality of polarized signals having amplitudes anddifferent wavelengths; b) controllable gain-control means, including adynamic filter for differential adjustment of the amplitudes of thepolarized signals, having a signal-input port connected to receivesignals from the signal source and a signal-output port for deliveringamplitude-controlled signals, the controllable gain-control means havinga control-input port through which a gain of the dynamic filter isadjustable for respective ones of the plurality of polarized signals; c)a length of signal transmission cable including an input port at one endand an output port at another end, the input port of the length ofsignal transmission cable being connected to receive signals from thesignal-output port of the controllable gain-control means, the length ofsignal transmission cable, in operation, changing polarization of thepolarized signals passing through it in accordance with the amplitudesand wavelengths of the polarized signals; d) a polarizing element havinga signal-input port connected to receive signals from the output port ofthe length of transmission cable and a signal-output port fordelivering, to a receiving device, signals conveyed from the length oftransmission cable; e) a signal monitoring device for determiningrespective states of polarization of the plurality of polarized signalspresent at its signal-input port which is connected to receive signalsfrom the signal-output port of the polarizing element, the signalmonitoring device having an output port connected to deliver, to thecontrol-input port of the controllable gain-control means, a result of adetermination which, in operation, it makes for the signals it receives;and f) the signal monitoring device and the controllable gain-controlmeans so operating together as to move the polarization states of thepolarized signals, by controlling the amplitudes of the polarizedsignals as they pass through the controllable gain-control means,towards alignment with a low-loss axis of the polarizing element.
 2. Thesignal transmission apparatus as claimed in claim 1, wherein thecontrollable gain-control means includes an intermediate power controldevice having an input port which serves as the control-input port ofthe controllable gain-control means, an optical amplifier and a dynamicgain control element which includes the dynamic filter, the intermediatepower control device being connected to control the optical amplifierand the dynamic gain control element in accordance with the signalsreceived from the signal monitoring device.
 3. The signal transmissionapparatus as claimed in claim 1, wherein the polarizing element hasattenuation on a high-loss axis which is greater, but not substantiallygreater, then attenuation on the low-loss axis.
 4. The signaltransmission apparatus as claimed in claim 1, wherein the signaltransmission cable is an optical transmission cable.
 5. The signaltransmission apparatus as claimed in claim 1, wherein the signal sourceis operative for generating return-to-zero signals.
 6. The signaltransmission apparatus as claimed in claim 1, and further comprising: afurther length of signal transmission cable including an input port atone end and an output port at another end; a further controllablegain-control means having a signal input port connected to receivesignals from the signal-output port of the polarizing element and asignal-output port for delivering further amplitude-controlled signalsto the input port of the length of signal transmission cable, thefurther controllable gain-control means having a control-input port; afurther polarizing element connected to receive signals from the outputport of the further length of signal transmission cable; and a furthersignal monitoring device for determining the respective states ofpolarization of the plurality of polarized signals present at itssignal-input port which is connected to receive signals form thesignal-output port of the further polarizing element, the signalmonitoring device having an output port connected to deliver, to thecontrol-input port of the further controllable gain-control means, aresult of a determination which, in operation, it makes for the signalsit receives.
 7. A method of transmitting a plurality of polarizedsignals having amplitudes and different wavelengths through a signaltransmission cable which changes polarization of the polarized signalsin correspondence with their amplitudes and wavelengths, comprising thesteps of: a) providing a dynamic filter capable of differentialadjustment of the amplitudes of the polarized signals and a polarizingelement in series with the signal transmission cable; and b) adjustingthe amplitudes of the polarized signals, at the dynamic filter, so thatpolarization states of the signals move towards alignment with alow-loss axis of the polarizing element.
 8. The method as claimed inclaim 7, including the step of monitoring the states of polarization ofthe polarized signals leaving the polarizing element and controlling theamplitudes of the polarized signals in accordance with results of themonitoring step.
 9. The method as claimed in claim 7, including thesteps of generating return-to-zero signals (RZ) signals and transmittingthe RZ signals.